DESCRIPTION (investigator's abstract): This application is for the renewal of two previous grants to study the regulation of B cell development. Our previous work has shown that the expression of Ig-gamma genes in developing B cells blocks their maturation and that substituting with the CH1 and transmembrane domains of mu Ig does not alleviate the block. It is planned to create and study another mu substitution and to investigate molecular/cellular events induced normally by signaling through a mu containing pre-B cell receptor. The expression of light chains is also tightly controlled in B cell development. Normally lambda1 light chains are in excess of lambda 2 and 3. In the SJL mouse strain, lambda1 expression is dramatically reduced. Previous findings suggest that a point mutation in the lambda1 constant region changing a glycine to a valine codon may be responsible. It is planned to knock-in a valine codon into a wildtype lambdal locus, If this change causes the defect, its molecular and cellular basis will be investigated by cell signaling experiments with various lambda1 mutant chains and by x-ray crystallographic comparison of the wildtype and SJL lambda light chains. Furthermore, based on unexpected results with a control serine knock-in mouse, the mechanism of hyper-activation of the lambda 1 locus by insertion of PGK-neo will be investigated. Understanding the development of B cells and its relationship to Ig gene rearrangement and Ig expression s of basic importance and clinical relevance. The regulation of Ig gene expression in B cells is one of the best studied systems of differentiation, Its complete unraveling will no doubt also give clues for the control of differentiation in general. On the clinical side, many immunological disorders involve B cells, such as autoimmunities, allergies, immunodeficiencies and, probably, susceptibility to cancer. Unraveling B cell development should help in understanding the basis of these diseases.